The invention relates to a braiding bobbin having a spool from which a fiber thread can be drawn off through a thread guide. The invention also relates to a braiding machine having several such braiding bobbins, and to a method for drawing off a fiber thread from a spool of a braiding bobbin.
Braiding machines normally make use of braiding bobbins with spools and thread guides with which the thread tension of the fibers is supposed to be kept as constant as possible during the braiding procedure. Particularly in the case of braiding machines in which very fine carbon fibers are processed, this procedure has to be very gentle on the fibers since otherwise the fibers can be damaged or even break.
Prior-art braiding bobbins differ substantially from each other in terms of the fiber spool employed. Braiding machines use either elongated fiber spools that hold quite a large volume of fiber material or else a short spool is used that can accommodate very fine fiber material, although only in smaller quantities. Both systems work in the same way in that, for example, an elongated spool with the previously wound-up fiber material is placed into the braiding bobbin and locked in place. The thread is guided through a thread guide that is positioned approximately in the center at the height of the fiber spool. From there, the thread is guided to a thread deflector at the foot of the bobbin, whereby this fiber deflector is integrated into a lever that is spring-loaded and that releases a ratchet at a given spring tension. This ratchet is located below the fiber spool and is joined to it by means of a clamping shaft. When the ratchet is released, the bobbin is allowed to rotate and the spool rotates under the fiber tension that is present. The rotation of the spool releases a length of fiber and the thread tension is abruptly reduced. The spring-loaded lever moves downwards, locking the ratchet again and thus also the fiber spool. As more fiber is drawn off, the procedure is repeated until the fiber tension has lifted the lever to such an extent that it releases the ratchet.
The course of the fiber tension has the form of a sawtooth. The thread tension rises relatively steeply as a function of the spring constant until the ratchet is released by the spring-loaded thread deflection lever. When the spool is released, it can roll freely over a catch and the thread tension drops briefly and steeply. The spool is then braked again until the next catch is released. When the bobbin moves from the outer radius of the bobbin curve towards the inner radius, the thread tension drops again as a function of the spring constant. This procedure is continuously repeated during the braiding procedure.
The spring-loaded lever and the associated fiber path are absolutely necessary for the braiding procedure because this is the only way in which the spring tension can be maintained at every point in time. The meandering course of the braiding bobbin in the braiding machine, however, gives rise to different fiber lengths from the fiber spool to the plaiting point. When the bobbin moves from the outer radius of the bobbin trajectory towards the inner radius, the fiber length decreases towards the braiding point. Without the thread compensation in the braiding bobbin, the thread would sag and be carried along by the other threads.
One drawback of elongated fiber spools is that the thread is drawn off at a slant above the wound-up fiber supply because of the short distance of the first thread guide. Consequently, in the case of sensitive fibers such as, for instance, carbon fibers, fiber damage occurs quite readily, building up and ultimately causing the fibers to break.
Therefore, in the case of sensitive fibers such as carbon fibers having a very low tex number, short spools are normally employed that allow a less problematic drawing off of the fibers. A disadvantage of shorter spools lies in the smaller amount of fiber that can be wound onto a spool. Since only very fine fibers can be processed with such spools, however, a sufficient length of fiber is available on the spool for the braiding procedure. Owing to the arrangement of the spool, however, the thread compensation path has to be solved in a different way. This is normally done by means of a multiple fiber deflector and a pulley principle provides sufficient fiber length for the length compensation. However, the large number of fiber deflectors inside the bobbin take-off has been found to be a drawback in such constructions. Typically, the thread passes four times through a fiber deflector of 180° and through an eyelet-shaped thread guide. Especially with the 180° fiber deflectors, fiber damage occurs that can then lead to fiber breaks.
For purposes of maintaining a predetermined fiber tension during braiding processes, it is likewise a known procedure to employ torsion springs inside a spool. For instance, European patent specification EP 0 402 526 B1 discloses a fiber-winding machine having a supply spool and an axle member with a coil spring inside it. The coil spring is operatively associated with the interior of the axle member and its outer end is in engagement with recessed portions located inside the drive part of the supply spool. When the axle member and the supply spool are rotated, the outer end of the coil spring is in engagement with one recessed portion in order to wind the spring up to a predetermined tension. Subsequently, continued rotation of the axle member and of the spool in the same direction causes the outer end of the spring to slip from one recessed portion to another to prevent overwinding or breaking of the spring. If sagging occurs during the fiber feed, the outer end of the spring is in engagement with a recessed portion and the axle member and the spool rotate in the opposite direction so as to wind the fiber onto the supply spool and to maintain a predetermined tension on the fiber.
Likewise known from the state of the art are various applications of brake devices that control the rotation of spools. For example, German Preliminary Published Application DE 1 435 219 A1 discloses a bobbin for a braiding machine in which several spools are mounted on a shared tension shaft in such a way that they can be rotated and braked. Here, the rotational axis of the spools coincides with the axis of the braking device. A spring that exerts a load on the brake assists the function of the brake.
The prior-art spool shapes of braiding bobbins, however, do not allow the processing of very fine carbon fibers without the occurrence of damage to the fibers. The dry fiber processing, along with the fiber-damaging draw-off, does not allow a reliable braiding process to be set up, especially when a large number of fiber spools are to be used at the same time.